1. Field of the Invention
The present invention relates to a dimming-control lighting apparatus for an incandescent electric lamp used in directing spaces such as a TV studio, a theater stage, or a photo studio, which uses a lighting device employing the incandescent electric lamp as a light source.
2. Description of the Related Art
In the related art, it is indispensable to freely control the flux of light of the lighting device in the directing spaces such as the TV studio or theater stage; as such, incandescent electric lamps have been widely used as the light sources of the lighting devices.
However, when the dimming of an incandescent electric lamp is adjusted, color temperature, as well as the flux of light, occurring in the incandescent electric lamp is changed, which results in a degraded directing effect in lighting.
In particular, in a TV studio when using a TV camera or a photo studio when photographing in color, the change of color temperature due to an adjustment of dimming adversely affects image quality. In order to solve these problems in the related art, various measures have been adopted including correcting the color temperature by mounting color filters at the front face of the lighting device, or controlling the luminance of an illuminated plane by adjusting the angle of illumination of the lighting device rather than adjusting the dimming thereof.
In general, the flux of light of a tungsten halogen lamp is proportional to the power of 3.38 of a supply voltage, and color temperature is proportional to the power of 0.36 of the supply voltage. Thus, color temperature is proportional to the power of 0.107 of the flux of light.
For example, in case of using the tungsten halogen lamp having a color temperature of 3200K when the rated voltage is supplied thereto, the color temperature is decreased to 2970K when the flux of light is adjusted to 50%.
FIG. 12 is a graph of the specific flux of light versus the color temperature when the tungsten halogen lamp is dimmed using a conventional dimmer.
The visual brightness (hereinafter referred to as LUX0) during stage lighting is regarded being proportional to the power of 2.0 to 3.0 of actual luminance (hereinafter referred to as LUX), and the control characteristic (called the dimming characteristic) of the dimmer is designed to conform to the proportional relationship.
For example, in a general case of using a dimming characteristic with the power of 2.3, when the dimming level becomes 50%, the LUX is about 20%, and the color temperature is proportional to the power of 0.245 of the LUX0, which is decreased to 2700K.
FIG. 13 is a graph illustrating color temperature change with respect to the LUX0 when the dimming characteristic has the power of 2.3.
As mentioned above, measures including using color filters or adjusting the light device or selecting the arrangement position of the same so as to avoid the decrease of color temperature have been adopted; however, these measures are complicated and troublesome, and cannot be regarded as complete methods for correcting the change of color temperature.
In order to solve the conventional problems, the applicant of the present invention has proposed a dimming-control lighting apparatus with less change of color temperature wherein a lighting device having a plurality of lamps or a lighting device having a lamp with two encapsulated filaments is used, and when the dimming is adjusted, one filament among the plurality of filaments first starts lighting and another one starts lighting when the color temperature of the one filament exceeds a predetermined value of a rated voltage. See Japanese Unexamined Patent Application Publication Nos. 2001-143880 and 2002-93588.
However, this lighting apparatus generally uses an AC power source, and requires control signal lines from the controller for controlling the dimmer.
On the other hand, most of the power sources for lighting apparatuses that have been installed in TV studios, photo studios, or theater stages are output power sources of the dimmer (hereinafter referred to as the dimming power source.) which are installed or constructed such that the dimming of the lighting apparatus can be freely controlled by the dimmer, and a connecting device, such as a connector, has been used so as to freely select and connect various lighting devices according to the purpose of its usage.
Thus, in order to use the above-mentioned dimming-control lighting apparatus for an incandescent electric lamp, new lines for a general AC power source and for control signals should be installed, which results in difficulty when employing incandescent in studios or theaters in which the lighting apparatus has already been installed.